I think it is too easy to split hairs here. The basic question is not out of line. Forget bickering about the definition of "expert" vs "advanced" vs whatever. The simple answer (roughly in line with what Bob P. said) is that with a week or two of really good instruction, someone who is fit and motivated can improve both their skills and their confidence dramatically. Whatever level someone ends up at, they will have a broader and deeper set of tools to handle different snow conditions - and thereby just plain have more fun.
A great deal depends on your time and money constraints. The "easy" road is to find a great instructor and book a bunch of privates. Someone with a reputation for both teaching excellence and technical skiing excellence. As others have noted, EpicSki is a good place to look. Based on my experience and talking with other people, the Ski Mag top 100 is the real deal as well. I have taken a number of privates from George Mosher at Targhee (consistently rated in the top 100). Every day, I have walked away with at least one major defect removed (well, reduced
: ) or one new tool in my tool-box. Had fun too. (note: bang for your buck, Targhee and George are a tough to beat combo vs. many of the really big resort ski schools). I think Targhee has a couple other people on the list this year as well. People rave about the famous Deb Armstrong's teaching and she is consistently on the same list - although, sadly she is currently ill. Every single EpicSki Academy attendee I've talked to has been thrilled. The key point is that with a bit of homework and planning, you can find a great learning situation.
Six half day or two-thirds day private or small-group lessons from a top instructor will certainly move your skiing up a few notches. Loosely, think in terms of 2 days learning/tuning a couple of basic skills you don't have. Two days dropping a couple of bad habbits. And two days learning skills for different conditions. If someone has the stamina for it, more days equal more learning.
I have been skiing 5-6 years. I am very, very far from what I (or any other rational person) would consider an "expert" skier. However, I can manage my way safely and comfortably on most ski area slopes under most conditions. And I have lots of fun doing it (maybe I'm an "expert" funner???). No way would that have happened without some excellent and focused multi-day instruction over the past few years.
Forget the dictionary hair-splitting. If you can afford it, and she'd really be into it, find an appropriate multiday lesson situation for your wife & give her a great gift. It'll almost certainly up the fun factor. That's what counts...